1. Field of the Invention
The present invention relates to a mobile station transfer control system in which a mobile station is able to utilize a plurality of cellular mobile communication systems service areas of which regionally overlap each other at least in part, a cell transfer control method, a mobile station, a cell transfer control method at a mobile station, a cell transfer control program, control apparatus, and an allocating method of communication resources.
2. Related Background Art
The following cell transfer method is among cell transfer methods of a mobile station in a cellular mobile communication system. A mobile station on standby or in communication in a certain cell monitors one or more surrounding cells as potential cell transfer targets. The monitoring of surrounding cells herein means measuring reception levels of pilot channels set for the respective cells. When a reception level of a certain pilot channel becomes higher over a predetermined value than the reception level of the channel on standby or in communication, the mobile station transfers to a cell corresponding to the pilot channel. Typical examples of the cellular mobile communication systems employing such cell transfer are the 800 MHz system and the 1500 MHz system in the digital automobile telephone system (Personal Digital Cellular: PDC) commercially served in Japan at present.
The PDC is generally operated in operation forms of operating the 800 MHz system and the 1500 MHz system independently of each other, but it is also possible to carry out a scheme of introducing a mobile station transferable between the systems and using the two available systems like one system. This scheme is sometimes substantiated in an operating form in which the aforementioned intersystem-transferable mobile station is allowed to use only cells located in some region out of those provided in a certain system (e.g., the 1500 MHz system) but not allowed to use cells located in the other region. Normally, a mobile station that can use only the 1500 MHz system is allowed to use all the cells.
In this operating form, when the intersystem-transferable mobile station on standby or in communication in a cell allowed to use in the 1500 MHz system moves into a region where only cells not allowed to use exist, control is sometimes carried out so as to make the mobile station transfer to a cell of another available system (e.g., the 800 MHz system), in order to maintain the standby or communication state. The following method is a potential example of execution of such control. Information on whether the intersystem-transferable mobile station can use the cell is broadcast in each cell of the 1500 MHz system. When the intersystem-transferable mobile station is notified of unavailability of a cell after transfer to the cell, it discontinues standby or communication in the pertinent cell and transfers to a cell of the other available system. This permits the intersystem-transferable mobile station to continue the standby or communication, without using the cell not allowed to use.
However, this method requires execution of a transfer process to a cell not used for standby or communication in practice, a synchronization establishing process, and a broad cast message receiving process, and thus raises concerns about increase in communication down time during these processes, increase in power consumption of the mobile station due to the processes, and so on.
In general, a cellular mobile communication system consists of a plurality of mobile stations, and a plurality of base stations each of which is in charge of one or more cells, and these cells constitute a service area in total. Each base station is connected through base-station control and cellular switching equipment to the public switched network. Each mobile station is allowed to establish communication within the service area while moving from cell to cell. As one of techniques applied to the case where a mobile station belonging to a certain cell is to transfer to another cell, there is a method in which the mobile station receives pilot channels from surrounding cells adjacent to the belonging cell of its own and monitors reception levels thereof. When a reception level of a pilot channel becomes higher over a predetermined value than the reception level of the communicating channel, the mobile station notifies the base-station control and/or the cellular switching equipment or the like (the network) of the fact. The base-station control or the like thus notified performs a transfer process for letting the mobile station transfer to a cell issuing the pilot channel of the higher reception level.
There exist a variety of such cellular mobile communication systems, depending upon their service contents, and according to circumstances, there are cases where a plurality of cellular mobile communication systems are simultaneously available in a same area. For example, there is an environment in which the 800 MHz digital automobile telephone system and the 1500 MHz digital automobile telephone system both are available. A scheme employed in this environment is a method in which a mobile station capable of operating in the both cellular mobile communication systems in a certain definite region can accept allocation of communication resources from either of the cellular mobile communication systems (a multi-system available scheme). The definite region is called a “common cell.” Specifically, in a cell set as a “common cell, ” the communication resources of the communication system thereof are allocated to a mobile station dedicated to the communication system of the cell and to a mobile station capable of operating in the both systems. In a cell (ordinary cell) not set as a “common cell,” the communication resources of the communication system thereof are allocated to only a mobile station dedicated to the communication system of the cell. By adopting this scheme, for example, in the case where the communication resources of one system (800 MHz) are running short and where the communication resources of the other system (1500 MHz) still have a margin, it is feasible to switch mobile stations using the system coming short of the resources, into the system with a margin of resources and thus utilize the communication resources effectively and flexibly. A system operator is allowed to determine which zone (cell) should be set as a “common cell,” cell by cell.
FIG. 1 is a functional block diagram for searching for a cell as a transfer target (handover destination) of a mobile station in the conventional multi-system available scheme. The processes illustrated in this functional block diagram are carried out by a control apparatus exercising control over each base station. First, input information received from a mobile station is fed into an input interface 701. This input information includes, for example, an identification number of the mobile station; cell identification codes and reception levels of pilot channels from surrounding cells about a cellular mobile communication system currently used for communication (the 1500 MHz system in the above example); cell identification codes and reception levels of pilot channels from surrounding cells about a cellular mobile communication system not used for communication (the 800 MHz system in the above example); and soon. When the mobile station is a device dedicated to either one cellular mobile communication system, the information about the other cellular mobile communication system is not included in the above-stated input information. Among the input information, the reception levels of the pilot channels are supplied to a cell information analyzer 702, and the cell identification codes to a cell code buffer 703. A cell code filter 704 selects a cell presenting the strongest reception level in the both cellular mobile communication systems and specifies it as a transfer target of the mobile station. A mobile station determiner 705 checks whether the mobile station can operate in the both cellular mobile communication systems. When the mobile station is one dedicated to one cellular mobile communication system, an output interface 708 issues a command to let the mobile station transfer to the cell specified by the cell code filter 704. When the mobile station is one capable of operating in the both cellular mobile communication systems, a common cell/ordinary cell determiner 706 further checks whether the cell as a transfer target is a common cell or an ordinary cell. An information storage 707 coupled to the common cell/ordinary cell determiner 706 stores information about which cell is a common cell. When the result of the check is that the cell as a transfer target is a common cell, the cell specified as a transfer target at the cell code filter 704 is approved as an actual transfer target and the output interface 708 issues an output to allow the transfer to the cell. When the result of the check is that the cell as a transfer target is not a common cell on the other hand, the transfer to the cell is not permitted, and thus the output interface 708 outputs information indicating that there exists no adequate cell. In this case, the mobile station is not allowed to achieve handover before the condition of the cell as a transfer target being a common cell is met.
The technology about the intersystem cell transfer method based on the direct monitor of the different cellular mobile communication systems as described above is disclosed, for example, in Japanese Patent Application “Laid-Open No. HEISEI 07-87544.”
In execution of such handover control, however, there arises disadvantage as described below with reference to FIG. 2. Let us suppose that the mobile station MS is a mobile station operable in both first and second cellular mobile communication systems and is under communication with the base station BS1 through use of the communication resources of the first cellular mobile communication system. In the direction of movement of the mobile station MS (on the right side in FIG. 2), there are a base station BS2 (ordinary cell) of the first cellular mobile communication system and a base station BS3 (common cell) of the second cellular mobile communication system. When the mobile station MS arrives at a site P and when a received field intensity from the common cell BS3 is not higher over a predetermined value than a received field intensity from the ordinary cell BS2, the cell of the base station BS2 of the first cellular mobile communication system now under use becomes a potential transfer target at the site P. However, since the cell of the base station BS2 is not a common cell, no handover is made at the mobile station MS operable in the both systems. When the mobile station MS further moves to a site Q, the received field intensity from the common cell BS3 becomes higher over the predetermined value than the received field intensity from the ordinary cell BS2, whereupon the mobile station MS is allowed to make handover to the common cell BS3. Incidentally, if the mobile station MS were a mobile station dedicated to the first cellular mobile communication system, the handover to the cell of the base station BS2 must be achieved at the time of arrival at the site P, based on the comparison between received field intensities from the two base stations BS1, BS2 in the first cellular mobile communication system. However, the handover is not made in practice before arrival at the site Q, because the mobile station MS is one operable in the both systems and the comparison of received field intensities is made between the different systems. As a consequence, there arises concern that the quality of communication (e.g., a reception level or the like) is not satisfactory during the movement of the mobile station MS from the site P to the site Q. Namely, in the case of the mobile station MS operable in the both systems, handover should also have been made at the site P in terms of use of the communication resources of the first cellular mobile communication system.
When the mobile station MS operable in the both systems arrives at the site P, if the received field intensity from the common cell BS3 is higher over the predetermined value than the received field intensity from the ordinary cell BS2, handover will be made to the common cell BS3 at the mobile station MS even if the received field intensity from the present cell BS1 is still strong, because BS3 is the common cell. Incidentally, supposing the mobile station were a mobile station dedicated to the first communication system, no handover would be achieved at the site P but handover must be made to the cell BS2 at the time of arrival at the site Q where the received field intensity from the cell BS2 becomes sufficiently high, based on the comparison between received field intensities from the both base stations BS1, BS2 in the first cellular mobile communication system. However, handover is achieved at the site P in practice, because the mobile station MS is one operable in the both systems and the comparison of received field intensities is made between the different systems. As a consequence, the mobile station MS abandons the communication resources of the first cellular mobile communication system that can be continuously used, and uses the other communication resources, during the movement of the mobile station MS from the site P to the site Q.
In each of the first and second cellular mobile communication systems, the system is designed to utilize the communication resources most effectively and efficiently; therefore, it is preferable to control the handover so that the mobile station operable in the both systems achieves handover under the same condition (i.e., at the same handover place) as the mobile station dedicated to the first cellular mobile communication system, while using the communication resources of the first cellular mobile communication system. However, in the case where the handover place is different from that of the mobile station dedicated to the first cellular mobile communication system as in the above example, there arises concern that the quality of reception degrades, for example, because of failure in ensuring the adequate distance between the cells using the same frequency.
An object of the present invention is to provide a mobile station transfer control system, a cell transfer control method, a mobile station, a cell transfer control method at a mobile station, a cell transfer control program, a control apparatus, and an allocating method of communication resources capable of solving the above problems under the environment where a plurality of cellular mobile communication systems are available, and making improvement in the cell transfer control of the mobile station.